Before strain gage-based load cells became the method of choice for industrial weighing applications, mechanical lever scales were widely used. Mechanical scales can weigh everything from pills to railroad cars and can do so accurately and reliably if they are properly calibrated and maintained. The method of operation can involve either the use of a weight balancing mechanism or the detection of the force developed by mechanical levers. The earliest, pre-strain gage force sensors included hydraulic and pneumatic designs. In 1843, English physicist Sir Charles Wheatstone devised a bridge circuit that could measure electrical resistances. The Wheatstone bridge circuit is ideal for measuring the resistance changes that occur in strain gages. Although the first bonded resistance wire strain gage was developed in the 1940s, it was not until modern electronics caught up that the new technology became technically and economically feasible. Since that time, however, strain gages have proliferated both as mechanical scale components and in stand-alone load cells. Today, except for certain laboratories where precision mechanical balances are still used, strain gage load cells dominate the weighing industry. Pneumatic load cells are sometimes used where intrinsic safety and hygiene are desired, and hydraulic load cells are considered in remote locations, as they do not require a power supply. Strain gage load cells offer accuracies from within 0.03% to 0.25% full scale and are suitable for almost all industrial applications.
The strain gage based load cell is a transducer that is used to convert a force into an electrical signal. The conversion happens in two stages. First, through a mechanical arrangement, the force deforms a strain gage. The strain gage measures deformation as an electrical signal because the strain changes the effective electrical resistance of the wire. Most load cells consist of four strain gages in a Wheatstone bridge configuration. The electrical output of the transducer is proportional to the mechanical force being applied.
Traditional strain gages work best when the contact points between the strain gage and the load can be controlled, such as is the case with traditional scales. Using strain gages in other applications can be problematic when the way in which the load contacts the strain gage is variable or uncontrollable. This has been the case when using a strain gage to measure the load on a rod passing through the strain gage. Since the deformation of the rod cannot be controlled, the rod can engage the strain gage in unpredictable way leading to errors in the produced measurement. What is needed is a strain gage for measuring the load on a rod that controls the contact points with the rod to produce more accurate measurements.